The prior art includes numerous multilamp photoflash arrays having a multitude of flashlamp sequencing arrangements. For example, series and parallel-connected lamp arrays, electrical and mechanical firing and switching configurations and relatively high and low voltage-responsive switches and photoflash lamps are known in the art. The switches also involve a number of distinct categories dependent upon the response to radiant energy such as melting, fusing or chemically reacting.
As the art progressed, the photoflash arrays have become smaller and more compact in order to reduce the cost and increase the efficiency. However, this miniaturization is not without problems, and one of the more evident problems is an increase in radiant energy applied to adjacent switches and, in turn, a tendency for an increased failure rate of the switches due to a condition known as "blow-off." As is known, the blow-off or burned off condition of the switching material occurs during conversion of the switch from a high to low resistance condition as the result of radiation from a nearby flashlamp. Moreover, it is also known that humidity has a deleterious effect upon radiation-responsive switches normally utilized in sequentially operated multilamp photoflash arrays.
Attempts to eliminate or at least reduce the above-mentioned problems have been and still are being provided. For example, U.S. Pat. No. 3,990,833 of Holub et al. suggests the addition of a humidity resistant organic polymer binder to the switch compostion to reduce humidity problems. Also, U.S. Pat. No. 4,087,233 of Shaffer provides for the addition of an oxidizer such as barium chromate which is particularly resistant to high relative humidity. Moreover, U.S. Pat. No. 3,969,066 of Smialek et al. adds cupric oxide to a switching composition along with a humidity resistant organic polymer binder.
Additionally, a blow-off reduction technique is provided in U.S. Pat. Nos. 4,156,269 and 4,164,007 wherein about 10% by weight of glass beads is added to the switching composition. This improvement was followed by a copending application bearing Ser. No. 21,398 filed Mar. 19, 1979, now U.S. Pat. No. 4,320,440, wherein inert filler particles such as titanium dioxide are included in the switch material composition.
However, the above-mentioned patents and applications are particularly addressed to high voltage arrays wherein a pulse voltage in the range of several thousand volts is employed to convert the switch from a relatively high to a relatively low resistance value. On the other hand, it can readily be understood that switches responsive to relatively low voltages, 10 to 20 volts for example, present other problems when conversion to a low resistance value is desired.
One suggestion for promoting good mechanical integrity and preventing "burn-off" in a low voltage switch is set forth in U.S. application bearing Ser. No. 253,358 filed Apr. 13, 1981, now U.S. Pat. No. 4,327,976, in the name of Shaffer et al. Therein, a powdered metal in stoichiometric excess is added to the switch composition and enhances the conductivity upon activation of the array from a low voltage source. Also, improved resitance to humidity is provided in an application bearing Ser. No. 148,358 filed in the name of Brower et al. on June 9, 1980, now U.S. Pat. No. 4,330,821. Therein, silver-coated glass beads are included in the switch material composition and serve to enhance the resistance of the composition to undesired humid conditions.
Although the above-described switch material utilizing silver-coated glass beads has provided excellent results in numerous applications and particularly in applications utilizing a relatively low voltage source, it has been found that there are other arrays and applications wherein such structures leave something to be desired. More specifically, it has been found that enhanced conductivity and resistance to humidity are desirable in switches responsive to a low voltage energizing potential.